The present invention relates to an optical information recording disk such as a write-once type optical disk, a rewritable type optical disk, and the like.
In a conventional optical information recording disk such as a write-once type optical disk, a transparent plastic plate is used as the substrate of the disk, and a recording layer, which is made of an organic pigment film, is formed on one surface of the disk substrate using a spin coating method or the like. A groove for guiding a light beam spot is formed in the recording surface. Information is recorded along the groove.
Information is recorded on the disk by causing a recording light beam spot to move along the guide groove. The guide groove is formed in a concentric or spiral pattern with a pitch between adjacent grooves of a small value such as 1.6 .mu.m. The alignment of the guide groove with the rotational center of the disk is critical in making the light beam spot accurately follow the guide groove correctly and stably.
In making the rotational center of the disk correspond to the center of the groove, the circularity of the center hole in the disk is an important factor. The center hole in conventional disks has been formed in a central portion of the plastic disk substrate by cutting, punching, or the like during the injection molding process. These techniques do not, however, consistently produce center holes with the desired degree of circularity.
In order to overcome this problem, there has been proposed an optical disk having a structure, as shown in FIG. 1, in which a hub member 4 formed of a lamination of metal plates is fitted into a center hole 3 in a disk substrate 1. The metal plate lamination hub member 4 is constituted by a resin hub member 6 having a center hole 8, and a metal plate 5 affixed to a flat surface of the hub member 6 and having a center opening 7 which acts as the center hole of the optical disk. Because the center hole which forms the center hole of the optical disk is formed by drilling, it can have an excellent circularity, and hence the eccentricity of the disk is significantly reduced. Moreover, use of the metal plate makes it possible to employ a magnetic clamper mechanism for clamping the disk on the turntable. In such a case, it is unnecessary to use in the clamper mechanism any member which presses the disk from the upper side thereof, thus making it possible to reduce the overall height of the recording/reproducing apparatus.
However, in the metal plate lamination hub member, the metal plate and the resin hub are a combination of an inorganic member and an organic member. It is of course difficult to join such dissimilar members with a high adhesive strength. Accordingly, there is a problem in that sometimes the components of the metal plate lamination hub member separate, causing failure of the hub member.
Furthermore, the metal plate used in the metal plate lamination hub member has been made of a stainless steel material, which is magnetic and resistant to rust. However, stainless steel typically has a Vickers hardness of only about 200, and hence the metal plate is relatively soft. Since the spindle is repeatedly inserted into the common center hole of the metal plate of the stainless steel plate and the resin hub member, there is a tendency for the outer diameter of the center hole in the metal plate to be enlarged irregularly, and hence the degree of circularity of the center hole in the metal plate is reduced.